The process that results in the oscillation of the vocal folds is actually somewhat complicated and may be tricky to grasp if you are "not good at physics", but let's give it a shot!
A somewhat vague explanation is that, due to the way the shape of the vocal tract causes air to flow through the glottis and past the vocal folds, the fluid pressure (where a fluid is a liquid or a gas) just above and just below the glottis is constantly shifting and either causing the vocal folds to come together or pushing them apart. This cycle of the vocal folds hitting together and being pushed apart happens over and over again, many times a second, creating a periodic oscillation or "vibration".
Some more details:
Early models of vocal fold oscillation relied on the Bernoulli Principle, which says that, as the velocity of a fluid increases, its pressure decreases. It was thought that the flow of air through the glottis caused the air pressure between the vocal folds to decrease, causing them to "collapse" together. The full closure of the glottis was then supposed to cause the air pressure below it to build up until it "blew" the folds open like saloon doors, allowing air to flow through it again until the whole cycle started over.
Later on researchers realized that this model was too simplistic and that these forces alone couldn't explain the continual oscillation of the folds, given the vibration-dampening properties of tissue.
The quote below explains how the model was refined to explain what really happens:
When the glottis is closing, the airflow begins to decrease, but the
air that is above the glottis does not "know" this, so it continues to
move with its same speed (because of inertia). This creates a region
just above the vocal folds where the air pressure decreases, because
air is not coming from the bottom through the glottis as fast as it is
leaving above. When the vocal folds are opening, fluid pressure
against the walls is greater than when the vocal folds are close
together. Thus, it is the asymmetry of driving force (air) that
The quote is from this website, which gives a more in-depth discussion on the topic and also provides some animated graphics to illustrate the various concepts involved. For example, it explains why the vocal folds don't just move together and apart as two uniform blocks but rather oscillate in a wave motion, as shown in this video.