By default, JMeter samplers are executed in order without any intervention. Timers are introduced to add wait to a thread group. Without delay there will be too many requests made to the server within a very short span of time causing the sever to choke. Timer will cause Jeter to pause for a certain amount of time before each sampler. If there are more than one timer added, JMeter will add up the time and will wait for the total time before executing the sampler to which the timer was applied. JMeter supports a number of timers like:

Constant Timer: This is one of the most popularly used timer in JMeter. This is used when we want each thread to wait for the same amount of time.

Uniform Random Timer
The uniform random timer causes each thread to wait for a random time.

Constant Throughput Timer
The constant throughput timer is a special type of timer used to create pauses with variable amount of time while maintaining the overall throughput i.e., samples/minute.

Bean Shell Timer
The Bean Shell timer is used to generate the delays using Bean Shell scripting.

BSF Timer
The BSF timer is used to generate delays using BSF scripting.

Gaussian Random Timer
The Gaussian random timer is used to generate delays using Gaussian distribution.

Synchronizing Timer
The Synchronizing timer is used to insert delay by blocking a certain number of threads. Once the blocked thread count reaches a specified number then the threads are released simultaneously thereby creating large load. As this timer generates large amount of instant load hence, it is favourable for spike testing.

Poisson Random Timer
The Poisson random timer is used to generate delays with most of the delay time interval occurring near a particular value.

JSR223 Timer
The JSR223 timer is used to generate the delays using JSR223 scripting.

Add a timer:
Right Click on Thread Group → Add → Timer → Select the required Timer