I'll start this question with the problem that led me to ask this question...

USNCO 2009 Problem 8 part c:

[![source:https: //www.acs.org/content/dam/acsorg/education/students/highschool/olympiad/pastexams/2009-usnco-exam-part-ii.pdf][1]][1]


Part c. states that all four of the compounds above are more basic than ammonia, and the solution explains that it is because the "carbon-containing groups" that are attached to the nitrogen are "better electron donors" than just hydrogen (in ammonia). This "inductive effect" causes the lone pair of the nitrogen to attract the proton ($\ce{H^+}$) better, or as the solution said, "donate the electron pair" to the proton better.

This got me thinking. 

My thought process was that the polarity of the hydrocarbon group and the nitrogen overall makes the atom extremely polar towards nitrogen. What I mean here is that the nitrogen has a large negative dipole. This also, in corollary, means that nitrogen has more electrons. In turn, it would attract the proton more strongly. 

However, I do not know if my thought process is correct. 

This is the official ACS solution to the problem in question:

[![source: https://www.acs.org/content/dam/acsorg/education/students/highschool/olympiad/pastexams/2009-usnco-exam-part-ii.pdf][2]][2]


  [1]: https://i.sstatic.net/BnBvP.png
  [2]: https://i.sstatic.net/KnvjY.png

What does it mean they are "better electron donors?" What is this inductive effect? If you can provide it, are there any other rules that are important pertaining to this and is there a source where I could learn these rules?

Thank you in advance.

Note: this is NOT homework, and my thought process for part c HAS been specified.

Source: https://www.acs.org/content/dam/acsorg/education/students/highschool/olympiad/pastexams/2009-usnco-exam-part-ii.pdf