Let's break the meaning down step by step:
A clause is basically a group of words put together with a verb phrase. When the words in the group convey a complete meaning and make sense, they are considered as "independent clauses". When the group of words need more words to complete their meaning, they are called subordinate dependent clauses. A non-finite clause is a dependent clause, first and foremost.
This means that this type of dependent clause consists on words that, when put together, convey a meaning that an action has not yet been performed nor completed. In other words, is similar to an infinitive.
An example of a non-finite clause would be: Saying "I love you" is the hardest thing for me to do.
The phrase "saying 'I love you'" does not make sense on its own, so it needs the rest of the sentence to make sense. That is what makes it a subordinate dependent clause.
Furthermore "saying 'I love you'" does not tell of a present, past, nor future tense. It is infinitive and it does not indicate something that has been done. Nobody has yet said "I love you" so it is out there "in the air".