Excerpt from Brackeys video tutorial.

MeshGenerator.cs

using UnityEngine;

[RequireComponent(typeof(MeshFilter))]
public class MeshGenerator : MonoBehaviour
{
    public int xSize = 20;
    public int zSize = 20;

    Mesh mesh;
    Vector3[] vertices;
    int[] triangles;

    #region Unity Method
    void Start()
    {
        mesh = new Mesh();
        GetComponent<MeshFilter>().mesh = mesh;

        CreateShape();
        UpdateMesh();
    }
    #endregion

    #region Editor Method
    void OnDrawGizmos()
    {
        if (vertices == null) return;

        for (int i = 0; i < vertices.Length; i++)
        {
            Gizmos.DrawSphere(vertices[i], .1f);
        }
    }
    #endregion

    #region Custom Method
    void CreateShape()
    {
        vertices = new Vector3[(xSize + 1) * (zSize + 1)];

        int i = 0;
        for (int z = 0; z <= zSize; z++)
        {
            for (int x = 0; x <= xSize; x++)
            {
                float y = Mathf.PerlinNoise(x * .3f, z * .3f) * 2f;
                vertices[i] = new Vector3(x, y, z);
                i++;
            }
        }
        
        triangles = new int[xSize * zSize * 6];

        int vert = 0;
        int tris = 0;
        for (int z = 0; z < zSize; z++)
        {
            for (int x = 0; x < xSize; x++)
            {
                triangles[tris + 0] = vert + 0;
                triangles[tris + 1] = vert + xSize + 1;
                triangles[tris + 2] = vert + 1;

                triangles[tris + 3] = vert + 1;
                triangles[tris + 4] = vert + xSize + 1;
                triangles[tris + 5] = vert + xSize + 2;

                vert++;
                tris += 6;
            }
            vert++;
        }
    }

    void UpdateMesh()
    {
        mesh.Clear();
        mesh.vertices = vertices;
        mesh.triangles = triangles;

        mesh.RecalculateNormals();
    }
    #endregion
}