At present, the way we count votes in elections (the voting system) means that the percentage of seats allocated to each party bears little resemblence to their percentage of the vote.
For example, in the last general election, the Tories got 36% of the votes, but 47% of the seats, while the Lib Dems got 23% of the vote, but just 9% of the seats.
The reason this happens is because instead of a general electon being a genuine 'national' election, it is in fact a collection of hundreds of 'mini' elections in a marked out area (each called a constituency) held across the whole country at 'generally' the same time.
The size of each constituency and their precise geographical position is controlled by our political masters, not by us the voters. The sizes and locations of these constituencies can have a bigger impact on the result than the actual votes cast - see gerrymander wheel. (It is important to note here that the constituencies can be absolutely equal in terms of the number of voters each have and yet still yield these perverse results in terms of seats allocated. So any argument that 'equalising' constituencies would solve the problem is false, constituencies are anyhow already fairly equal in size in the vast majority of cases).
Until we allocate seats in 'proportion' to votes cast we will continue to live in a 'semi' democracy where results are manipulated by the political elite. The best way to demonstrate why this is important is with a simple mathematical model:-
Party A wins 40% of the vote and has policies x, y-20 and z+3
Party B wins 35% of the vote and has policies -x, y+20 and z-1
Party C wins 25% of the vote and has policies -x, y+10 and z-2
The policies can represent anything you want them to, for example x could be to introduce a DNA database or maybe to invade Iran or re-introduce the poll tax, with -x signifying opposition to these. y could represent how much to alter taxes, redistribute wealth or alter the deficit, z could be the age at which we are entitled to vote or stand for parliament, or age of consent etc. You get the general idea.
In the above example, under our present voting system, party A most likely wins outright winning more than 50% of the seats (although any one party or none could win depending on how the constituencies are positioned and how many there are). This would enable party A to try to implement policy x despite the majority 60% explicitly voting for the opposite -x, decrease y by 20 despite the majority voting for an increase and increase z by 3 despite the majority voting for a decrease. I think you get the general idea of why this is undemocratic.
Under a proportional voting system a coalition would have to be formed, almost certainly between the parties with the most similar policies, this would mean the majority would get policies much more in tune with what they voted for, -x, an increase in y and a decrease in z. You can alter the variables and percentages of vote as much as you like and still the proportional system will always deliver the majority of voters more of the policies they voted for. Try it yourself.