In 60-card constructed formats, you can run up to four copies of most cards — meaning your best cards show up reliably, game after game. Commander flips that: it's a singleton format, so you get exactly one copy of almost everything.
That single change ripples through every part of deckbuilding. You can't just run "four copies of your best removal spell" — you need several different removal spells that each do a similar job. Consistency has to come from effect density and redundancy of role, not literal card copies.
A constructed deck asks "how many copies of my best card can I fit." A Commander deck asks "how many different cards can I find that do roughly the same job."
— the core deckbuilding shift between formats
Constructed deck limits come from rule 100.2a; the Commander singleton rule comes from rule 903.5b.
Comprehensive Rules · Rule 100.2a
"In constructed play... each deck has a minimum deck size of 60 cards. A constructed deck may contain any number of basic land cards and no more than four of any card with a particular English name other than basic land cards."
Comprehensive Rules · Rule 903.5b
"Except for basic land cards, no two cards in the deck may have the same English name."
How to take advantage of it
Tutors are premium in singleton. A card that searches your library for a specific answer is far more valuable in Commander than in a four-copy format, since it's the only reliable way to guarantee access to your single copy of something.
Think in categories, not specific cards. When deckbuilding, ask "how many cheap removal spells / how many draw effects / how many ramp pieces do I have," not "do I have four of my favorite one."
Redundancy of effect beats redundancy of card. Since you can't run four Sol Rings, look for several different cheap mana rocks that all do a similar job instead.