Hi @tcsabina
You will find your answers on our docs.
Here are listed few pages:
https://docs.sonarqube.org/latest/user-guide/concepts/
→ Technical debt : The estimated time required to fix all Maintainability Issues / code smells
https://docs.sonarqube.org/latest/user-guide/metric-definitions/
Technical Debt ( sqale_index )
Effort to fix all Code Smells. The measure is stored in minutes in the database. An 8-hour day is assumed when values are shown in days.
Technical Debt on New Code ( new_technical_debt )
Effort to fix all Code Smells raised for the first time on New Code.
Technical Debt Ratio ( sqale_debt_ratio )
Ratio between the cost to develop the software and the cost to fix it. The Technical Debt Ratio formula is:
Remediation cost / Development cost
Which can be restated as:
Remediation cost / (Cost to develop 1 line of code * Number of lines of code)
The value of the cost to develop a line of code is 0.06 days.
Technical Debt Ratio on New Code ( new_sqale_debt_ratio )
Ratio between the cost to develop the code changed on New Code and the cost of the issues linked to it.
Enjoy the reading 
Carine