Big Letters: another name for UPPERCASE.
Also, Capitalisation: use of Uppercase Letters to begin certain words in written text, the remainder being in lowercase.
The purpose of this is to draw attention to important words - typically Proper Nouns - and to help mark the division of text into sentences. The first word of a sentence begins with a capital letter, so it's easy to orient oneself when moving through the text.
some people miss the point of capitalisation, and from lazyness or a strange affectation, they insist on writing only in lowercase. this makes it hard to scan the text quickly and to tell where one sentence ends and the next begins. thus their writing is hard to read and rapidly becomes irritating, detracting from the impact of whatever information the individual in question is trying to communicate. so please don't do it. please.