Rwanda is one of a small number of countries to have changed its flag not because the government changed but because of what the old flag had come to mean.
Why the 1962 flag had to go
The flag Rwanda flew from independence in 1962 was a vertical tricolor of green, yellow, and red with a large black "R" in the center. The letter was there for a practical reason: without it the flag was nearly identical to Guinea's. What ended the design, though, was association rather than confusion. By 2001 those colors were bound up with the period of division that ended in the 1994 genocide against the Tutsi, and a government trying to rebuild could not keep flying them.
The replacement was adopted on October 25, 2001, alongside a broader set of reforms aimed at reconciliation. Alphonse Kirimobenecyo designed it, and the brief was to point forward rather than back.
Blue over yellow over green
The blue band takes the top half of the flag, with narrower yellow and green bands below it. A golden sun with 24 rays sits in the upper right of the blue field.
The readings are direct. Blue is happiness and peace, yellow economic development, green hope and prosperity, and the sun enlightenment and transparency. Every element was picked to describe where the country intended to go, which is what the commission asked for.
In use
The flag flies at government institutions, national events, and international gatherings, and the government sets out how it is to be displayed and handled. It appears at Independence Day celebrations and at memorials. Schools teach its symbolism directly, part of the same campaign that produced it.
Comparisons
Horizontal stripes and a sun emblem are common enough in African flag design, and Rwanda's draws on that vocabulary. What sets it apart is the reason for it. The 2001 flag has drawn attention from people who study flags precisely because it was a deliberate attempt to use a national symbol as an instrument of post-conflict recovery.
References
[1] Rwanda's Ministry of Culture and Sports - Official Documents Link
[2] BBC News - Rwanda Flag Change Historical Context Link