What does 'lo' mean?
What does 'lo' mean as in 'es lo mismo' and 'no lo sé'.
2 Answers
The short answer is that it changes depending on context which doesn't help you much but I've attached an explanation that hopefully will make some sense 
For your given examples:
es lo mismo - it's 'the' same
no lo sé - I don't know ('it', ie whatever was being talked about before)
Lo is one of those words that doesn't always have a clear definition — and it can function in at least three different ways, as a subject pronoun, object pronoun or definite article. When you run across the word in a sentence and don't know what it means, you often need to figure out first how it is being used.
As a masculine direct-object pronoun: In such cases, lo can be translated as either "him" or "it." The feminine equivalent is la.
As a neuter definite article: The definite articles in Spanish, typically el and la when singular, are the equivalent of the English "the." Lo can be used as a neuter definite article before an adjective to make an abstract noun. For example, lo importante can be translated as "the important thing," "that which is important" or "what is important."
As a neuter direct-object pronoun: Lo can be used as an object pronoun to refer to something abstract, to an unnamed activity or situation, or to a previous statement:
No podemos hacerlo. We can't do it. No lo comprendo. I don't understand it.
But is it possible to just say: 'no comprendo' or do you have to say 'no lo comprendo'?
Is it like this: No comprendo = i don't understand
No lo comprendo = i don't understand it
