Fifteen year old thread with comments within the last few months, and no answer, god damn it internet. Well, for future searchers of the phrase, here you go.

A spline is a mathematically defined curve. If you've ever used an art tool that allows you to draw with vector shapes (the kind that have two points that mark the ends and then two 'handles' that define the direction of the curve at those ends), you've come across splines. Those ones are Bézier splines, but there are other types with different maths behind them that serve different purposes. Google the phrase 'flat spline' if you want to see the nifty mechanical device that the word comes from, but nowadays (and especially in a programming context) it's about the mathematically defined curve.

The term 'reticulate' seems to have a broader meaning, but it means something along the lines of 'to divide something into smaller connected parts'.

Because modern graphics hardware deals mostly with straight lines and triangles, you can't draw a curve like a spline 'directly' - you render it by evaluating the position at lots of points along the spline, and then drawing short straight line segments to connect those dots (kind of like how a game can approximate a sphere with lots of triangles). So reticulating a spline would be the process of evaluating the points along a spline, so you've broken it up into short line segments to render it.

That's what it would mean if it meant anything, but actually it was a joke phrase from old Maxis games that became a meme, and now you'll see it pop up in modern software as a reference to Sim City.