The p command will take the most recent thing you have in your register and place, put, or paste it where ever you want.
Say you delete a whole line by hitting dd. After that you can move your cursor anywhere else and if you press lowercase p it will place that line you deleted below the line you have your cursor.
If you delete a whole line like in the above example, using the capital P will place it above the line.
If you just select a word, or a block of text that isn't a full line:
Lowercase p will place to the right of the cursor and capital P will place to the left of the cursor.