A Plank is a great exercise to develop a stable core. Having the ability to switch on a strong, stable core is crucial for your lower back health as well as general athletic ability. The trunk needs to have the ability to stiffen so you can lift heavy things or quickly move your arms or legs without your back buckling under the load.
To be safe when you start doing this exercise make the bridge slightly higher (more hunched as this is a safer position for your lower back (a too-low bridge with an arched lower back can lead to excessive loading of the lumbar sacral junction and lumbar facet joints).
To perform the plank position yourself in a push-up position then contract your gluts/but so that your pelvis curls “under” and the small of your back flattens out. Then keep your core “switched on tight” throughout holding this position.
Many people can hold the plank for 30 sec or so straight from the beginning however over time people often are able to build up to a 60 or even 90 sec while maintaining this active holding position.