Click and drag to make a selection box. With the Select tool active, click and drag to create a temporary box around the items you want to select. When you release the mouse button, your items become selected. If you click and drag to the left (a crossing selection), anything completely or partially inside the box becomes selected. If you click and drag to the right (a window selection, shown the figure), you select only what’s completely inside the box.
Can't say it better than this text from the sketchup help site https://help.sketchup.com/en/article/3000085 or visio's help site https://support.office.com/en-us/article/Select-shapes-6853abb0-54c2-4069-8634-2a7d34bc3c1b#bm2
Click and drag to make a selection box. With the Select tool active, click and drag to create a temporary box around the items you want to select. When you release the mouse button, your items become selected. If you click and drag to the left (a crossing selection), anything completely or partially inside the box becomes selected. If you click and drag to the right (a window selection, shown the figure), you select only what’s completely inside the box.