At the basic level, Liquid comprises of two different markups - Output and Tags.
Output markups can be used to display things on to your webpage. The information can be from a variable or a placeholder or an object reference. They can also be used to create filters for advanced output.

{{ }}

{{ portal.contact_info }}

Tags are logical statements that let you program your theme the way you want. They can be used to create comments, conditional statements, loops etc. inside your code, like you do in a typical programming language like Java.
Using the “if” block to do something if you have any (more than 0) Forum topics in your portal

{% if portal.topics_count > 0 %}

<!--Do Something Cool-->

{% endif %}

Running through each solution category in your portal and doing something accordingly

{% for category in portal.solution_categories}

<!--Do Something-->

{% endfor %}

Next: Dynamic Placeholders