This sign, on the pedestrian crossing near my house, is on red. Red means do not cross. It could be unsafe as there might be traffic coming that could hurt you. Badly.
When the sign turns green it is safe to proceed as any cars should have stopped to let you cross.  Usually there is a beeping noise when the sign is green which makes you want to hurry across.
Sometimes if they think there are no cars coming people are tempted to cross when the sign is on red. This is a personal decision. No responsibility is accepted here if you are run over.
Occasionally people press the button and cross before the sign has changed to green. This tends to annoy drivers who may find themselves sat there waiting when there do not appear to be any pedestrians wanting to cross. Most of us will be guilty of this.  Ah well.