 Object Mode is the default mode of Blender, and is helpful for manipulating objects as a whole. Objects can be moved around and animated throughout the scene. However, what this object looks like is determined by its mesh data, and mesh data can be edited through Edit Mode. Edit Mode is the default mode of the modeling workspace, but you can also access it by simply changing this drop-down menu here in the top left-hand corner. For hotkey users, you can press Tab to toggle between Edit and Object Mode. While in Edit Mode, you will be able to manipulate vertices, edges and faces to change the shape of your mesh. Once finished, you'll be able to toggle back into Object Mode to move the object as a whole once again. It's helpful to know that your Edit Mode history and the action of toggling into Edit Mode all count as a step in your Undo History. Therefore, after you finish an Edit and go back into Object Mode, Undoing your steps will correctly bring you back into Edit Mode with all of your Edit Mode history intact as well. For example, I'm just going to change this cube into a random shape by selecting one or multiple vertices and using the transformation tools to move, rotate or scale the vertices around. Then we'll toggle back into Object Mode with Tab. Just to top things off, let's rotate our cube to add an action in Object Mode as well. Now let's say I changed my mind about something and want to go back in my Undo History. Simply hit Ctrl Z to Undo and you'll see that our Edit Mode Undo History is still intact and accessible. Fun fact, cameras, lights, light probes, empties, images, speakers and force fields all do not have an Edit Mode. But there are non-mesh objects such as armatures, curves and lattices that do have an Edit Mode accessible but may look a little different. For these objects, you won't be manipulating mesh data but rather the object-related data that they might have. However, the principles of editing these objects remains the same. Manipulate the building blocks to create what you need. Blender can also edit multiple mesh objects at once. To do this, simply select all the objects you would like to edit and go into Edit Mode. You'll notice that you'll be able to manipulate vertices, edges and faces of any of the objects you have selected simultaneously. However, you won't be able to have any of the objects mesh data interact with each other as trying to connect vertices between objects will not work. For example, normally you can press F as in fill to join two vertices with an edge but because these two vertices belong to separate objects, Blender restricts me from connecting them. In order to actually connect these vertices, I would have to join these two objects into one mesh. To do this, simply select them in Object Mode, go to the Object menu and select Join. For hotkey users, you can simply press Ctrl J. Once you've done this, you can now edit them in Edit Mode with no restrictions. In case you want to separate your mesh again into different objects, you can go into Edit Mode, select a part of your mesh, go to the Mesh menu and select Separate. Here you have a few options but selection is the one you want. This will separate your selected geometry into a new object with its own mesh data.