What is meant by "logical language", is that the language's grammar is engineered so that any statement you make is syntactically unambiguous. Grammatically correct statements only parse one way.
In natural languages, and many constructed languages, the meaning of a statement can be interpreted to be a number of things, depending on context.
An example to given in the "Complete Lojban Language" book (CLL), is the term "Pretty Little Girls’ School". In English, there's no way to tell what this means. Does it mean a school for "pretty little girls", a pretty school for "little girls", or a small school that is pretty and for girls?
Lojban's grammar is such that the relationship between words is unambiguous. The meaning to words themselves doesn't matter all too much to the language's designers, just the relation between those words. As such, ambiguity is still possible, but usually in the form of semantic vagueness.
That said, it's not like there's only one way to say anything. According to the CLL, there are Forty ways to translate the English phrase "Pretty Little Girls' School" into Lojban. Among them, is
melbi cmalu nixli ckule, which would be a direct translation of the English phrase, word-for-word, and refer a "school for girls who are beautifully small". Whatever "beautifully small" means.
But if you go through the list of possible interpretations, you'll notice that there are in fact some statements that evaluate to the same meaning, either because that's how the semantics work out, or because the two statements can be logically equivalent. (
2 + 2 = 4 is equivalent to
2 × 2 = 4 and
2² = 4)
Ultimately, these properties are what make Lojban a tempting tool to use for communicating with AI, who are notoriously bad at working out vague context we use in everyday speech. And it's also why Lojban is called a "Logical Language".