As a JVM application, the Elasticsearch main server process only utilizes memory devoted to the JVM. The required memory may depend on the JVM used (32- or 64-bit). The memory used by JVM usually consists of:

as a Java application ElasticSearch uses memory allocated to java applications obeviously

Heap space in Java is used for dynamic memory allocation for Java objects and JRE classes at the runtime. New objects are always created in heap space and the references to this objects are stored in stack memory.

heap space are configured via -Xms and -Xmx in mbs

sudo nano /etc/elasticsearch/jvm.options

Modify the value of -Xms and -Xmx

Best practise , dont put more that 45% of your RAM

# Xms represents the initial heap space
# Xmx represents the maximum heap space

Here set values as below

-Xms256m
-Xmx256m

Now restart Elastic Search

sudo systemctl restart elasticsearch