 The knife tool is a modeling tool you can use to form new edge loops and vertices. To use it, simply go into the modeling workspace or go into edit mode, then select the knife tool from the quick tools menu on the left. From there, you can either left click drag or tap left click to begin cutting into your mesh. You may notice at this time that there is still a purplish-pinkish line attached to your cursor after drawing. To disengage this knife tool drawing mode, simply double left click or press E. This will then allow you to continue drawing if you so choose. However, I do warn you not to right click during this stage as it will cancel the cuts you have just made. To fully confirm the cuts you have made, simply press space bar or enter. As you can see, the visual style of the edge loops and vertices you have drawn have now matched the original mesh. This means they are fully applied. For further details about the knife tool, simply reference the bottom bar of the UI while you have the knife tool activated. Now, if you've ever wanted to cut anything in half, similar to a cross-section, you would want to use the bisect tool, one of the knife tool's subtools. To use it, simply left click drag the knife tool in the quick tools menu on the left. This will open up a sub-menu that includes bisect. Hover over the bisect option and release your left click. From here, you can simply draw a line or cut across your mesh. This will represent where you want to cut your mesh in half. Afterwards, a gizmo should appear that will allow you to adjust your cut. However, the most important settings are in the operator panel in the bottom left-hand corner. Simply expand this and adjust the settings as you see fit. But most importantly, these checkboxes will allow you to fill the face that you cut open, as well as select which side of the cut you would like to keep. Both the knife tool and the bisect tool can be very helpful for editing your mesh.