A rectangular concrete block measures 50 cm by 50 cm. Calculate the total area of 12 such blocks in square meters.

One concrete block is $50 \text{ cm} \times 50 \text{ cm}$.
Since $1 \text{ m} = 100 \text{ cm}$, we convert $50 \text{ cm}$ to meters:

$$50 \text{ cm} = \frac{50}{100} \text{ m} = 0.5 \text{ m}$$

Using the formula Area = side $\times$ side:

$$\text{Area of one block} = 0.5 \text{ m} \times 0.5 \text{ m} = 0.25 \text{ m}^2$$

Thabiso needs 12 blocks.

$$\text{Total area} = 12 \times 0.25 \text{ m}^2 = 3 \text{ m}^2$$

The total area of the 12 concrete blocks is $\boxed{\text{3 m}^2}$.