You're asking about meter in poetry. Meter is a rhythm created by a pattern of stressed and unstressed syllables. Not words. Syllables.
Let's use some of the words in your question to illustrate:
The word "really" has two syllables. When you say that word out loud, you give a little extra push or emphasis or stress to the first syllable. You say "REAL ly," not "real LY."
When you say the word "explanation," you stress the first and third syllables -- "EX pla NA shun." Try saying it with any other stress pattern -- "ex PLA na SHUN," for example -- and you'll hear how wrong that sounds.
When you say "example," you stress the middle syllable -- "ex AM pul."
For any word with more than one syllable, you can identify the stressed syllables by saying the word out loud and listening for where you naturally put the extra emphasis. When a line of poetry has a lot of one-syllable words, you have to say the whole line out loud to hear which words naturally get stressed. For example, the first line of William Shakespeare's play Twelfth Night has ten syllables and only nine words. There are eight one-syllable words in that line, and just one two-syllable word:
If music be the food of love, play on.
When you say that line out loud, you can hear how the stresses fall:
if MU sic BE the FOOD of LOVE play ON
Try stressing the line any other way, and you'll hear that it doesn't sound right.
(None of those one-syllable words is naturally stressed or unstressed by itself. Words that are stressed in that sentence might be unstressed in a different sentence. Each word is stressed or unstressed because of the way it fits together with the words around it.)
When you're writing your poem, just keep saying the lines out loud. You'll be able to hear the stress patterns.