Straighforward Play is just what the name implies; playing the hand's strength directly.
Weak hands fold.
Hands with a decent chance to be the best will bet.
Strong hands with a high probability of being best will bet and raise.
Weak hands which have a good chance of drawing to be best will call.
That's all there is to it. No bluffing, semi-bluffing, check-raising, SlowPlay, etc.
The majority of the time, straightforward play is correct. There are three major areas where people deviate incorrectly from straightforward play:
They fail to bet or raise stronger hands. This is because most people are passive players.
They make a lot of bad calls. They either call with draws that are too weak to warrant a call, or make calls with weak pairs when they are almost certainly beaten by others.
They have FancyPlaySyndrome and feel compelled to resort to unnecessary tactics.
You shouldn't deviate from straightforward play unless the alternative will make more money; this is usually not the case. Far too many people feel compelled to deviate just because they can.