What is meant by "crowding-out effect"? How is it related to planning investment ?

The crowding-out effect is a possible effect of increased government spending on private investment. In some circumstances, increased spending by the government can reduce the funds available for private investment---in theory even so much that it actually results in the same total amount of investment, and simply a shift from private to public spending.

In practice, crowding-out is rarely if ever total in this way; it's more likely that say 30% or 50% of the additional government spending is offset by reductions in private investment.

The usually hypothesized mechanism is via interest rates: Increased government spending requires increased government borrowing, which drives up equilibrium interest rates until they become too high for private investors to want to take out loans to finance investment. (After all, you're not going to want to invest in a project that pays a lower return than the interest rate you can get on bonds.)

But this is actually quite easy to offset with monetary policy---print more money to bring interest rates back down. The real constraint is actually productive capacity. There is only so much wealth a country can produce at any given time, and while this capacity is usually not fully utilized, a sudden large increase in government spending could actually use it all, and thus make it unavailable for private use; the best example of this is WW2, where the industrial capacity of every major country in the world was fully utilized making war materiel. During WW2 there was indeed a great deal of crowding-out, and even ostensibly free-market countries like the US and UK rationed goods extensively--ranging from sugar to steel. Hardly any civilian vehicles were produced in the US during this period, because the factories were used for building tanks and artillery.

At an individual level, crowding-out is honestly not that important to worry about. There are so many other factors affecting your risk and return on investment that the possibility of a sudden shift to expansionary fiscal policy should be the least of your worries. Yes, theoretically that could happen, causing inflation or driving up interest rates---but it's just as likely that a competitor will scoop your investment, or the project will fail (as most startups do), or there will be a global price shock on some raw material you need (oil is the obvious example, but the same could easily happen with lithium, neodymium, or even helium). In fact, the most likely cause of expansionary fiscal policy would be a severe recession, and in such circumstances you'd want that expansionary fiscal policy, because crowding-out will be minimal and the spending will primarily go to restoring full employment.