There will be things which many modern readers may not understand without going and looking it up, for example:
Dialects: Dickens frequently has characters use class or regional dialects, with their own ‘not standard English’ vocabularies. Often the less familiar words will be explained for the reader in the text. e.g in David Copperfield Mr. Peggotty describes David and Em’ly as being 'like two young mavishes', the older David who is narrating explains for us that he 'knew this meant, in our local dialect, like two young thrushes, and received it as a compliment'.
It is also likely that many readers would still know that ‘mavis’ is a mostly obsolete term for the bird we all call a ‘thrush’ these days.
Elsewhere we may be left to decipher a dialect or less formal speech register for ourselves, for example in Bleak House, Jo says:
‘And she ses to me' do it,' and I dun it, and she giv me a sov'ring and hooked it’.
Most British readers would have no difficulty mentally translating this to
‘And she says to me ‘do it’, and I done it, and she give me a sovereign and hooked it’
and in turn interpreting that as
‘And she said to me ‘do it,’ and I did it, and she gave me a sovereign and left’.
Matters are confused by Jo using a present tense when speaking about an event in the past, but I interpret this as narrator’s licence, giving an immediacy to his telling, and this is still a feature in British English and there is probably a technical name for it, of which I am unaware. Edit: @GarethRees advises in comments that this is the Historical Present tense which Wikipedia also calls the Dramatic Present or Narrative Present.
If native speakers aren’t familiar with ‘hooked it’ meaning left, they may still be familiar with ‘playing hooky’ as a term meaning playing truant from school, or ‘to sling one’s hook’ meaning ‘to go away’.
People might get tripped up by an earlier reference in the same passage to ‘by a lady in a wale’, but once you become familiar with the fact that Dickens often gives his Cockney characters the habit of replacing ‘v’ with ‘w’, and you read it phonetically, you get ‘a lady in a veil’. The V/W switch for Cockneys is a subject of much debate, Dickens wasn’t the only writer to describe this, but it is difficult to know if what they were hearing was cockneys making a sound that was neither quite V now W or something else was going on.
Regardless of the origin, if you come across words like ‘wery’ and ‘wittles’ in Dickens, they mean ‘very’ and ‘victuals’.
Current Events: where Dickens references events and concerns which were current in his time. For example, on this Stack we’ve had a couple of recent questions which required a bit of unpicking of Dicken’s personal views on Prison and Justice Reform. Some people will know about that, some people will make a guess and miss Dickens actual intended meaning (in answering one of those questions I initially went down a bit of a blind alley myself because I thought I knew what Dicken’s views were, and it turned out I didn’t), though these will tend to occur in more polemic passages and not impinge on understanding of the main plot.
Otherwise, sometimes Dickens’ vocabulary is no longer fully current, he might make reference to types of carriage, articles of clothing, occupations, currency denominations, legal instruments, foodstuffs and other items which now require a little more than general knowledge to fully understand, but these are seldom crucial to the plot.
English changes according to time, place and station in life of the user. We are looking back at Dickens through a lens over 100 years thick, most of us probably need to use some degree of external resource to fully comprehend every passage.
What resources one might find useful depends which book you are reading and what aspect of it you are having trouble with. In any case there is no defined set of resources I am aware of to point you to. Work through the text methodically and when you have worked out what kind of problem you have having, search for resources accordingly.