The speaker begins "London" by telling us a little story. He wanders through each "charter'd street" that happens to be "near where the charter'd Thames does flow."
Hmm, seems a little repetitive doesn't it? True, but this is probably because the speaker really wants to emphasize this whole "charter'd" business.
Speaking of which, that little word can mean a number of different things. In this context, it has the sense of "confined" or "mapped out" or "legally defined."
Hmm, what do we mean by "legally defined"? Well, "charter" often refers to a document issued by a government or political official that grants certain rights or privileges, defines an entity, that sort of thing.
In these lines "charter'd" evokes all of these different senses. The speaker is suggesting that the streets of London, and even the Thames itself (the river that flows through London), are increasingly the subject of government control.
Alternatively, they are increasingly constricted, rigidly defined—in other words, not "open" or "free."
Now we should tell you that, in lots and lots of Blake's poems, both in the Songs of Innocence and Experience and elsewhere, constriction, narrowness, and the government are usually not the greatest of things. Blake is always about openness, freedom, imagination.
To summarize then: the speaker wanders through London, and notices that something is amiss.
(History note: Just in case you wanted to know, here's an idea of what the Thames may have looked like in Blake's day.)