OpenShot uses Titles to create images to overlay on videos.  The images OpenShot creates have transparent backgrounds and include text or additional pictures.  OpenShot includes several templates with the Basic title editor, and you can download and install Inkscape to edit the templates or create your titles.

Click the Title menu in the menu bar at the top of the main window, then click the Title option.  Select one of the templates by left-clicking the icon on the left side of the window.  Fill in the template fields on the right side of the screen.  You can enter the File Name and the text to display and select the Text Color and Background Color.  Do not change the Background Color if you want to create a transparent overlay.

Changing the font of a title in OpenShot is done in the basic editor by clicking the Change Font button then selecting a font other than the default.  You can change the font, style, and size in the Change Font window.  If you want more control of the overlay created, you can click the Advanced Editor button to launch Inkscape.  Install Inkscape and ensure that you configure OpenShot to use Inkscape as the Advanced Editor.  Using Inkscape, you can use most fonts installed on your system to create a title overlay.  To ensure that the font you use appears in OpenShot, you may convert the text to an object using paths.  Once the text is an object, you can save your file and close Inkscape, and the image will i/mport to OpenShot as a title.  Whether you use the Basic or Advanced Title editor, your Title appears in the Project Files list with the name you provided in the template after you click the Save button.  Here is a link to a video tutorial demonstrating how to use the basic and advanced editors in OpenShot.

You can also use any image manipulation program (GIMP, Photoshop, Inkscape, etc.) to create an image (preferably an SVG or PNG) with a transparent background.  Add text that you want with the fonts installed on your system to your image.  After creating the image, import it to OpenShot and add it to your timeline on a track higher than your video.