a starter is boiling a little DME/LME in some water for about 10 minutes, letting it cool, then pitching the yeast in it and seal it up. Some people use a growler to do this, I use a pyrex pint or quart measuring glass and cover the top with saran wrap. The idea is that you give the yeast a head start on multiplying before you pitch it into the wort so fermentation starts sooner. Some do their starters days in advance, I usually do it a few hours ahead of time.
Yes, you could just pitch the yeast into the 70 degree wort, it would just take a while to start (like it's doing now).