The Schengen area is a part of Europe and also includes several overseas territories. Within it, you can freely cross borders at any point in the member states without stopping.
The Slovak Republic is part of the Schengen area. By coming to Slovakia, you therefore also enter the Schengen area.
You can read about your Schengen area travel options in the following articles.
Member States of the Schengen area introduce border controls in emergency situations. Currently, there are border controls at the borders of Slovakia with the Czech Republic and Austria.