How can the computer keeps track of the seconds?

Because of this

that’s quartz crystal. Quarts has a property called Piezoelectricity. Crystals with this property generate a charge when pressed or modify their shape when charge is applied to them.

This property allows you to create a Crystal oscillator which oscillates in regular intervals.

Each computer has several crystal oscillators generating different pulses which drive the computer.

To know when a second passes, you only need to know the frequency (i.e. number of oscillations per second) of that crystal. You then simply count these oscillations.

Modern computers have special chips for this. They are commonly called Real-time clocks. These chips contain everything necessary to keep track of time: oscillator and counter. They are also battery-backed so they continue to run even if the computer is switched-off and disconnected from power.