There are two requirements for the secure use of symmetric encryption:

First thing is to have a strong encryption algorithm, reason  is that  if hacker has access to a few encrypted texts and algorithms, he should not be able to decrypt the information or figure out the key. 
In a brief, hacker 
should not be able to decrypt the text even if he possessed cyphertext and algorithm.

In fig 3.1 

If  X is plain text
E is an encryption algorithm and 
K is key

Cyphertext Y = E(X,K) 

Even if someone has access to ciphertext(Y) and Encryption Algorithm(E) He should not be able to deduce Y.

The second thing is sender and receiver must share their copies of the key in a secure fashion and should not disclose the key. 
Because if someone has known the key as well as the algorithm, he can simply reverse the algorithm and all the information is readable.

Plain text(X) = D(Y,K).