。
There are a lot of words I don't understand.
Literal
Understand-not words [subject-が] many exist.
A relative clause: 分からない ('don't understand') modifies 言葉 ('words'). No relative pronoun like 'which' or 'that' is used — Japanese just places the modifier directly in front of the noun. たくさん is the adverb 'many/lots,' modifying ある. Note the use of ある (existence verb for inanimate things) — abstract things like words also take ある, not いる.