17 Seconds gives you useful info quickly.
If you think about a trademark as a “mark” of a “trade,” then it makes more sense.
Consider the origins of branding. Farmers needed a way to identify their cows from their neighbor’s cows, so they branded (i.e. burned) their unique brand (mark) into their cows (trade). If one brand looks too much like another, then the brands are not doing their job, and we can’t tell whose cow is whose.
And if one trademark is too close to another – and if the goods are also too similar – then there is a “likelihood of confusion” between the two marks.
By the way, trademark lawyers often use “marks” and “trademarks” synonymously. Ditto for “trademarks” and “servicemarks.”