'Forcing' means encouraging the bulbs to bloom out of season. Generally it involves planting the bulbs in a container somewhat shallowly (compared to in the ground), storing in a protected area during winter and bringing into the warmth and light early so that the bulbs bloom ahead of schedule. Some bulbs are suited to forcing by growing in just water although they tend to be a one-time use abd are disposed of after bloom. Not all bulbs are well adapted to forcing, hence the notation in the catalog.

If you are just planting them in the ground, you can ignore this. Or if you want to experiment with forcing, there are many online instructions to help you.

Here is a link that might be useful: spring bulb forcing fact sheet