Welcome to the MongoDB Community @nisajthani!
RDBMS knowledge/theory is not required to work with MongoDB, and for some learners MongoDB may be the first database system they use. My overall recommendation would be to learn more about the database management system(s) you plan to use, so you can best take advantage of the features and strengths each offers.
Some fundamental database concepts such as indexing are common across different database implementations, but others (such as schema design patterns and query optimisation) will have different considerations given MongoDB’s more flexible schema support. Even though all database systems support some form of indexing for data retrieval, available index types (compound, geospatial, text, …) and data size limitations will vary.
MongoDB is a distributed database including automatic failover, so more advanced deployment approaches like replication and sharding will also differ from the RDBMS equivalent (depending on which RDBMS system(s) you are using as a reference). For one example comparison, see Comparing MongoDB vs PostgreSQL.
The free online courses at MongoDB University are a great starting point for MongoDB knowledge, and courses like M001 (MongoDB Basics) do not presume you have any database experience. There are learning paths with recommended courses for either Developer or Database Administrator topics.