Almost every command takes a count
You already saw 5j and 3w. The principle scales: a leading number tells Vim "do this command N times." The count integrates with the operator-motion grammar — counts can sit on either side, or both, of an operator.
Counts on motions
5j down five lines. 3w three words forward. 10l ten characters right. 4) four sentences forward. 2} two paragraphs forward. Anywhere a motion goes, a count multiplies it.
Counts on edits
3dd delete three lines. 5x delete five characters. 2yy yank two lines. 4>> indent four lines. 10p paste the last yank ten times in a row.
Counts on insertions — the surprise
This one always delights newcomers: in Insert mode, you can prefix the entry command with a count, and Vim repeats whatever you typed. 10i-<Esc> inserts ten dashes. 5oTODO<Esc> opens five new lines, each containing TODO. 3aword<Esc> appends "word" three times.
The grammar with two counts
3d2w delete six words. 2c3l change six characters. The two counts multiply. In practice you'll usually pick one count and put it on whichever side feels natural.
relativenumber (Track 4) and the gutter shows the distance from your cursor. Want to delete down to that line? Read the number off the gutter — say it's 7 — and type 7dd. Vim becomes a video game where targets have their distances printed on them.