The CIA triad of Confidentiality, Integrity, Availability refers to three necessary security components of software, at a bare baseline.
Confidentiality
Software(and its’ developers) are often exposed to sensitive details. These details should only reach the people the client expects. Users must be able to understand what data can be seen by who.
Integrity
Data should only be changed purposefully by users. Data, along with corresponding metadata, should be stored safely, securely, and with solid backups.
Availability
Access to your service and the data is paramount for users. Redundancy, fail over, backups, disaster recovery, and rigorously maintained infrastructure are all important parts of continuing availability.
So why is this important? Every software developer needs to have have strong knowledge of security – writing software without that knowledge is irresponsible because the consequences to software providers and users. Any tiny bit of leaked data, email address, user name, can be used as a ‘wedge’ in getting more data.
OWASP is the best source for information related to your specific technology stack and implementation. Please go through and read each of their recommendations for every application you write.
This is important because the deck is stacked against you: you have to defend from every possible attack, attackers only have to find one weakness.