A context-free grammar is ambiguous if some word has two different parse trees. Often this is undesired: for example, we would like $$a + b \times c$$ to be read as $$a + (b \times c)$$ rather than $$(a+b) \times c$$.
Some languages are inherently ambiguous, that is, any context-free grammar for the language will be ambiguous. The standard example is $$\{a^nb^nc^m\} \cup \{a^nb^mc^m\}$$.