New to Mathcad

First, does anyone know what this means I understand it is some kind of function. The deltas are displacements.

We cannot say much from a small picture, but at least we can see that you are using Mathcad 15 or below and that you have View Regions turned on 😉

Give it a try - open your sheet, click on the Phi in your expression and press F1 - you will get help for the Heaviside function. Phi is Mathcad's implementation of the Heaviside Step function. That functions returns zero for negative values and +1 for positive values. There are different implementations as to what should happen if the argument is zero - Mathcad returns 0.5 in that case.

So the expression you show returns +1 if Delta is greater than Delta0. Yo can see it as the step is moved so it does not occur at Zero but at Delta0.

Most of us learned to use the software by doing. The built-in help with its Tutorials, Quicksheets and E-Books is a good place to start and many of us have learned a lot by simply listening to the discussions in the old collab forum and now here in this community.