I feel like I am missing something - from what it seems, JSP comes out of the box with support for tags, as this question's answer shows (the guy was asking a pure-jsp question and got an answer involving tags). But if I try to run the given code
<c:out value="${myString}"/>
(with myString defined before, of course), the jsp just writes the above line into the html.
Do I have to do something extra to enable it?
Jakarta Tags (JSTL) support is dependent on the server/servletcontainer used. Some ships with JSTL, others don't. This is regardless of the JSP/Servlet version. Usually, normal JEE servers such as WildFly/Payara/TomEE already ship with JSTL out the box, but barebones servletcontainers such as Tomcat/Jetty/Undertow don't. For them you'll need to install JSTL yourself.
It's actually pretty simple:
Install JSTL JAR files as per instructions in this answer: How to install JSTL? It fails with "The absolute uri cannot be resolved" or "Unable to find taglib" or NoClassDefFoundError or ClassCastException.
Declare the <c:xxx> tag namespace in top of JSP as per the JSTL documentation (click any of the taglibs to see the declaration examples). For JSTL core it's the following:
<%@ taglib prefix="c" uri="jakarta.tags.core" %>
or if you're still using JSTL 2.0 or 1.2, then it's:
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
You may notice that some poor online tutorials would suggest to extract the JAR file and clutter the webapp's web.xml with the TLD declarations. You should never do that, this is a wrong suggestion which is caused by the change in taglib URI's during the JSTL 1.0 -> JSTL 1.1 step. Instead of updating the taglib URI's in JSP, ones decided to redefine the old taglib URI's in web.xml and it became a myth.
JSP itself ships with only the <jsp:xxx> tags out of the box. These are not part of JSTL.