The 'slant' is actually describing the shank that the presser feet and needles attach to, different machines have either low, high or slant shanks. There are some others that have special attachments- like bernina's. All 301's are slant shank (it's not an option), so if you are looking for accessories make sure that they say 301 or slant shank on them. It was supposed to make it easier to see your sewing.
A singer featherweight 221 would be a 'straight' low shank, and needs different attachments.