If you grow plants, often you can propagate the plant with cuttings. Tomatoes are a great example.
In the photo I have indicated in red where "suckers" exist. These stems will not produce fruit on this plant but will take energy and nutrition that could go to produce more and larger fruits. So they should be pruned (cut) from the plant.
If you want more of that plants of that kind of tomato, you can take that cutting and either place it in water or in soil and it will root. Now you have another tomato plant. A simply and inexpensive way to expand your garden.
If you go ahead and place the cutting in soil, just keep the soil moist until the cutting roots. That is what I typically do. But you can keep the cutting alive by placing it in a container with water also - of course you will eventually want to plant it unless you do hydroponics.
This is a Cherry Tomato I pruned today although I don't need more plants of this variety. Again, pruning results in more fruit and larger fruit so learn which stems are "suckers" and cut them away.
A sucker will grow between the main stem and a branch or limb. It sort of grows in the "V" where the branch attaches to the main stem and it will not produce fruit. So cut it away.