Strength

Strength (str) is a measurement for...potential. Each part in the game has a str-rating, so each build has as well. Putting more or more powerful parts on a chassis raises the total strength of the build.
Additionally the Rating effects str as well, having a very low rating (under the default 1500) lowers it, while a high rating increases it. Thus the builds of players with a very high rating (<1900 e.g.) have a lot more str then the same build of someone with a low rating.
Weight also affects str, 1200 kg is '0', +/- 45 kg is 1 str each, since spammers/rammers draw their advantage from weight.
Strength matters for Quick Matches (QMs), as only players (or bots) with about the same str are matched, as well as Daily Beatings Achievements.
Note: The str in the upper left corner is sometimes off (prob. due to mods). Always check str on the profile page! Especially if QM wins don't count for the achieves.