A suffix is something added to the end of a word to change its meaning. In the case of -less, we add this to a noun to turn it into an adjective. The meaning of -less is without. For example, the adjective homeless means without a home. Here are a number of examples with sentences:
Powerless: After a natural disaster in another country, many people feel powerless to help.
Clueless: He has no idea what's happening; he's completely clueless!
Useless: We try to avoid buying useless gadgets just because they're in fashion.
One of my favorite adjectives with -less is relentless, which means without stopping.