Can I use dfault .crt and .key for ssl in my newly purchased domain and hosting from bigrock?

See more:

server

web-hosting

SSL

So I just baught a new domain and single domain linux hosting from bigrock and I checked in hosting thet have a ssl folder in root directory. This folder has following contents in it. I haven't baught SSL certificate from them

/ssl/
/ssl/certs/<filename>.crt
/ssl/csrs/
/ssl/keys/<filename>.key
/ssl/ssl.db
/ssl/ssl.db.cache

I am new to SSL but since I have .cer and .key files I wants to know if I can use them to implement SSL in my domain?

If your answer is a "YES" then how can I do that?

If "NO" then is there any free SSL certificate issuing authority? Please let me know

In both the cases I want to implement SSL on my site.

What I have tried:

Well I did some study but I am unable to understand what exactly they are telling. A few of them start telling about some linux commands to implement openssl and some configration in nginx or apache server.

I am not looking anything like that. I simply purchased hosting and domain and implemented a wordpress site in that and now I wants to implement a free SSL (If possible)

During my search I came to know that Let's Encrypt[^] provides some free certifications but I am unable to understand How?

Please share some noob guide if you have one.