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From this page:
Software timers in a nutshell
A software timer (or just a 'timer') allows a function to be executed at a set
time in the future. The function executed by the timer is called the timer's
callback function. The time between a timer being started, and its callback
function being executed, is called the timer's period. Put simply, the timer's
callback function is executed when the timer's period expires.
Note, a software timer must be explicitly created before it can be used.
Efficiency considerations in software timer implementations
Software timer functionality is easy to implement, but difficult to implement
efficiently. The FreeRTOS implementation does not execute timer callback
functions from an interrupt context, does not consume any processing time
unless a timer has actually expired, does not add any processing overhead to the
tick interrupt, and does not walk any link list structures while interrupts are
disabled.
The timer service task (primarily) makes use of existing FreeRTOS features,
allowing timer functionality to be added to an application with minimal impact
on the size of the application's executable binary.
Important information on writing timer callback functions
Timer callback functions execute in the context of the timer service task. It
is therefore essential that timer callback functions never attempt to
block. For example, a timer callback function must not call vTaskDelay(),
vTaskDelayUntil(), or specify a non zero block time when accessing a queue or a
semaphore.
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