Answer:
The basics of proton-proton fusion in the Sun are detailed in the following important summary:
Normally, protons repel each other because their charges are similar. This is similar to trying to bring together the north pole of a magnet with the north pole of another magnet.
To overcome that electromagnetic repulsion, one needs to smash the protons at a very high speed. This is similar to how two magnets can be brought together if they are moving very fast.
That high speed is not achieved in daily life, thankfully, but in the cores of stars where the temperature is high.
Temperature is a proxy for the speed of particles. For example, if it is cold in a room, the particles are moving slowly.
The temperature is high in the cores of stars because there is the sizable mass of all the overlaying layers exerting a pressure on the core. This pressure causes the temperature to rise, and hence the speed of the protons.
By analogy, consider when diving from the top of the pool to the bottom of the pool. As you descend, you begin to feel the pressure exerted by all the overlaying layers of water.
In the core of the Sun, the temperature is about 15 million degrees Celsius. This is hot enough for the protons to move at very high speeds. When two protons collide at high speed, they can fuse together to form a helium nucleus. This process releases a large amount of energy, which is what powers the Sun.
The proton-proton fusion reaction is a complex process, but it is essential for the Sun to shine. Without this reaction, the Sun would eventually cool and collapse.